It is known that the crossing of particular land areas by military vehicles can be interdicted by distributing a plurality of land mines in the area. Each mine is capable of independently fuzing upon the approach of military vehicles. Two types of such mines which operate on magnetic principles are known.
Mines of the passive type operate in the mangnetostatic mode: they sense the distortion of the earth's magnetic field by the approach of a vehicle, or they may sense any magnetization residual in the vehicle itself. They ordinarily have a ferromagnetic-core sensing mechanism to increase their sensitivity, and operate with very low battery drain and hence remain operative for extended periods. In addition, they are subject to countermeasure techniques, and also are triggered by vehicle side-passes at distances where the resulting discharge does not damage the vehicle and is thus wasted.
Mines of the active type operate by creating a local electromagnetic field and detecting distortion of that field caused by the approach of a vehicle. They are less subject to side pass difficulties and to countermeasures, but require so much energy that their batteries quickly discharge and the effective life of the mine is intolerably reduced.
A further desirable characteristic for land mines should be mentioned. For direct overpasses, a mine is more effective beneath the front axle and cab portion of a wheeled vehicle, but is more effective beneath the center of the tank: accordingly, it is desirable that a mine discharge at an optimum point in the overpass of a vehicle, depending on the nature of the vehicle.